It is well known that jet aircraft which perform at high altitudes are subject to flameout at which altitude it is not possible to obtain emergency on board electrical and hydraulic power from auxiliary power units (APUs) until the aircraft has descended to a lower altitude at which the air breathing APU may be started. APUs are used for providing power to start a jet aircraft on the ground and further to provide on board power in the air when the jet engine has flamed out. Existing emergency power units (EPUs) are provided in jet aircraft to provide emergency on board power for altitudes above the altitude at which the APU will operate. U.S. Pat. Nos. 3,722,217, 3,800,534 and 4,033,115 disclose systems for providing emergency hydraulic power. These EPUs use stored chemical energy systems which have fuels such as hydrazine for providing emergency on board power. As is known a jet aircraft has propulsive power supplied by the jet engine(s) to maintain flight velocity and on board power which is electrical and hydraulic power for controlling the control surfaces and electrical loads of the aircraft Unfortunately, these chemical based EPUs impose costs and handling penalties as a result of the toxicity characteristics of the chemical energy system fuels.
Currently, and in the future, aircraft which are designed to operate at high altitudes require uninterruptible on board electrical and hydraulic power over their entire altitude flight envelope The aforementioned chemical based EPUs represent a substantial cost and handling penalty in aircraft which are to achieve high performance. Accordingly, a need exists for an EPU having minimal cost and performance penalties for use in transient high altitude emergency power situations wherein flameout can occur at an altitude where APUs are not operational.
FIG. 1 illustrates a typical supersonic aircraft flight envelope As illustrated therein, above 40,000 ft., the APU is not operational to provide any power for supplying emergency on board hydraulic or electrical power. In this region, the aforementioned EPUs such as those involving hydrazine or other chemical energy storage systems have been utilized prior to the present invention. In the region between 30,000 and 40,000 ft., the APU is operational with a reduced output which is insufficient to satisfy the necessary on board hydraulic and electrical requirements necessary to maintain control of the aircraft when a flameout occurs. As illustrated, below 30,000 ft., the APU is operational to supply the totality of electrical and hydraulic on board power necessary to maintain control of the aircraft during flameout and to restart the aircraft
Conventional two spool turbofan jet engines have a power takeoff from the high speed spool. Power has been extracted from the low speed spool for running oil lubrication pumps. The high speed spool is mechanically connected to an airframe mounted accessory drive which provides both electrical and hydraulic on board power for the airframe. However, the prior art to date does not utilize a power takeoff from the low speed spool during high altitude loss of propulsive power to generate hydraulic and electrical on board power to control the airframe until it has descended to an altitude low enough to permit the engines of the aircraft to be restarted or for on board power to be provided by the APU.
U.S. Pat. No. 2,952,973 discloses the generation of auxiliary power to operate a fuel pump, hydraulic fluid pump and an alternator during high speed operation and to operate the aforementioned fuel pump, hydraulic pump and alternator by the low pressure turbine at low aircraft speeds. However, the '973 patent does not teach the generation of emergency power by the aircraft during flameout or other emergency engine operation conditions
U.S. Pat. No. 3,662,975 teaches the generation of emergency power from an auxiliary compressed gas source even in the event of loss of power of all jet engines by compressed gas applied to the power generating turbine.
U.S. Pat. No. 3,834,161 teaches the generation of auxiliary power from a free power turbine. However, the '161 patent does not teach the generation of emergency power from a power takeoff connected to the low speed spool of a jet engine